Kia's fourth-generation Optima is an ambitious car. It's the most polished, best-equipped model in the range, one that will go toe to toe with mid-sized rivals such as the Toyota Camry, Mazda6, Subaru Liberty and Volkswagen Passat.

The Si features 17-inch wheels, a 7.0-inch touchscreen system with satellite navigation, Bluetooth connectivity and six-speaker stereo along with cloth trim and tech such as autonomous emergency braking and lane departure warning systems, front and rear parking sensors and LED fog lamps.

The top model also features a more comprehensive driver assistance suite that adds active cruise control, rear cross traffic alert and a blind spot detection systems to the standard model's automatic braking and lane departure warning systems.

It even has an inductive charging pad for compatible smartphones, allowing customers to simply plonk their phone down in the right place for wireless battery charging.

2015 Kia Optima GT Photo: Supplied

As with every model in the Kia range, the new Optima comes with a seven year warranty, capped price servicing and roadside assistance package

It's a spacious machine on the inside, thanks to a new body that is 10mm taller and longer than its predecessor, and 25mm wider too.

There's a decent amount of headroom and legroom in the front and rear, and a wide range of seat position and steering column adjustment for drivers.

2015 Kia Optima GT Photo: Supplied

The GT version we tested was particularly comfortable with eight-way powered adjustment for the driver and passenger, and a memory-setting function that allows drivers of different sizes to store preferred settings. It's an impressive pace to be, trimmed in black leather with red stitching, where quality soft-touch plastics meet black high-gloss trim in the Si and aluminium trim in the GT.

The cabin is nicely laid out with BMW-inspired entertainment and ventilation controls set into neat horizontal rows below a bright and reactive touchscreen. The cabin ambiance in Kia's premium version is helped by GT logos on the front seats and a steering wheel – a sporty, flat-bottomed number trimmed in perforated leather with shift paddles tucked neatly behind.

It has USB and 12-volt power outlets in the front and rear, along with plenty of places to stash gadgets. There's a good amount of space in the boot too – 510 litres of storage room that should be more than enough for most customers.

Under the bonnet, the Si's 2.4 litre engine remains an effective if unremarkable option, providing adequate momentum while using a little more fuel than ideal.

Quiet at low engine speeds but slightly coarse in its upper reaches, the naturally aspirated motor makes 138kW and 241Nm, outputs that reach the front wheels through a six-speed automatic transmission.

The GT uses the same driveline hardware, but feels much stronger thanks to its turbocharged motor.

The 2.0-litre unit makes a healthy 180kW of power, an amount backed up by a solid 350Nm of torque. It is a smooth and powerful engine that makes excellent progress on the road, the automatic transmission doing a fine job of keeping it on the boil.

But this isn't a sports car, and it doesn't feel like a high-performance engine. It's missing the zinging urgency of something like a similarly powerful Subaru WRX or Volkswagen Golf GTI, feeling brisk but not thrilling.

It's a similar case on the road, where the GT's firmer, more performance-oriented shock absorbers lend it a mature, competent character without imbuing genuine athleticism. Body roll is well-controlled and it rides well thanks to the brand's local work to optimist its cars for Australian conditions.

Unlike most manufacturers, Kia has elected to run with separate steering systems for the two models.

The regular Si uses a column-mounted power steering system, while the Turbo has a more expensive rack-mounted system for better feel and accuracy. The GT's steering feels a little heavier and more precise, though there's nothing wrong with the regular system in the Si.

Though they look purposeful, red-painted brake callipers on the GT grab the same 320mm ventilated front discs as fitted to the Si.

In a welcome development for the brand, both models benefit from excellent tyres in 215mm-wide Continentals on the Si and premium 235mm-wide Michelin Pilot Sport rubber for the GT.

It's a well-sorted machine, one that deserves consideration from four-door buyers.

But Kia will have to work hard to convince buyers to drop more than $40,000 on a mid-sized sedan.

That's the next step in its success plan – replicating the sales and marketing success of its Hyundai sister brand, which has been the second-highest marque for sales some months this year.

That's probably the toughest item on its to-do list, but Kia's recent wins suggest the brand will get there soon.